The nuclear reaction in a power generating nuclear reactor of the boiling water type, also referred to as a boiling water reactor, is controlled by a set of hydraulically actuated bottom-entry control rods. The control rod drive pistons enter the pressure vessel of the reactor through vertical penetrations in the lower plenum of the reactor known as control rod drive housings. These housings extend below the pressure vessel of the reactor far enough to provide the pistons sufficient travel so that the control rods may be completely withdrawn from the reactor fuel core.
At the bottom of each housing, the hydraulic actuation mechanisms for each piston are bolted to the housings with flange connections. These flange connections are typically secured by eight socket head capscrew bolts that are symmetrically located around the control rod drive flange.
The hydraulic actuation lines, sometimes referred to as insert/withdraw lines, enter the flange assembly through ports which are typically located on the top surface of the control rod drive housing flange.
Control room reactor operators must know the vertical position of each control rod at all times. Magnets located on the bottom of the control rod drive pistons, when sensed by an electronic probe, provide this information to the operator for each control rod. These electronic sensors, or position indicating probes, send their signals to the control room through the probe cables which are plugged into probe housings at the bottom of each control rod drive.
The flange assemblies contain a number of hydraulic components which relate to the actuation of the control rod drive pistons. During use, reactor coolant is employed as the working fluid flowing through these hydraulic components. Particulate debris consisting of neutron-activated corrosion products and wear fragments, as well as the water soluble radioactive contaminants that result from normal plant operations tend to accumulate in the hydraulic system. The particulate debris is from the alloys that are present in the various piping systems and internal components in a boiling water reactor. The water soluble radioactive contaminants carried by the reactor coolant through the flange assemblies are either fission products which have leached through the small perforations of failed fuel rods or materials that become radioactive through neutron-activation.
None of the materials used in the piping or other reactor structural components are radioactive when they are installed. However, as a result of normal use, these structures and components will tend to accumulate radioactive contaminants, and will eventually become sources of radiation themselves. Many of the materials used in the piping and structural components are subject to corrosion, friction and wear, and these processes release metallic particulates into the reactor coolant. The reactor coolant carries these particulates throughout the entire system. While the reactor is operating, a very dense flux of neutrons is present in the reactor's fuel core, and the particulates and contaminants carried by the coolant are exposed to this neutron flux. When a high-energy neutron collides with an atom of any material, the atom is typically transmuted into a new species, and in nearly all cases the transmuted atom is unstable or radioactive. This process is known as neutron-activation. It is through neutron activation such as this that the particulates and contaminants in the reactor coolant are made radioactive. Eventually, the particulates and contaminants in the coolant accumulate in regions of slow flow or plate-out onto attractive surfaces which are contaminated with radioactivity in this manner. Because these particulates and contaminants are often carried through the reactor fuel core many times before settling or plating-out, nearly all of them have experienced neutron activation. It is through this cycle of (1) corrosion, friction or wear related deposition into the reactor coolant; (2) neutron activation; and (3) sedimentation, accumulation or plate-out that the piping and other reactor components eventually become contaminated with radioactive materials.
The neutron activation process in a nuclear reactor of the boiling water type produces a great number of radioactive nuclides. Most of these radioactive nuclides are derived from the constituent elements of stainless-steel or any other alloy present, such as brass, if brass is present in the condenser tubing of the reactor. Typical nuclides are the several radioactive isotopes of Cobalt, Copper, Iron, Manganese, Nickel, Tin and Zinc. The radioactive water soluble contaminants will typically consist mostly of the various radioactive isotopes of Cesium and Iodine. However, in practice, one single nuclide, Cobalt-60 or Co-60, dominates the total radiation produced by all others to such a degree that practical experience has shown that shielding can be designed substantially as if that were the only nuclide present.
Cobalt is often used in industrial applications to increase the toughness of various alloys. Essentially all of the naturally occurring Cobalt is the stable and non-radioactive isotope Co-59. When Co-59 is carried by the Reactor Coolant through the fuel core the neutron activation process typically yields the radioactive nuclide Co-60. Decay of Co-60 emits radiation in the form of a simultaneous pair of gamma photons with energies of 1.17 MeV and 1.33 MeV, and its half-life is roughly 5.25 years. Metallic lead is an excellent shielding material for this type of radiation.
As these radioactive materials accumulate on the interior surfaces of the reactor vessel, the control rod drive housing and the flange assemblies, these components themselves become intense sources of radiation of all forms, i.e., alpha particles, beta particles and gamma photons. In addition, the outer surfaces of these components can become contaminated from other sources, such as the release of radioactive contaminants caused by the deluge of water from an adjacent control rod drive. In practice, however, only gamma radiation, dominated by the two high-energy Co-60 photons, can penetrate the thick stainless-steel walls of the flange assembly. The resulting contact dose rates for these components, even months after reactor shutdown, routinely range as high as ten Roentgen Equivalent Man (10 rem) per hour and can be substantially higher in certain cases. A Roentgen Equivalent Man (rem) is the most common unit employed to measure radiation dose rates and is usually defined as that quantity of any type of ionizing radiation which when absorbed by man, produces an effect equivalent to the absorption by man of one Roentgen of X- or gamma radiation. A Roentgen is that quantity of X- or gamma radiation such that the associated corpuscular emission per 0.001293 grams of air produces, in air, ions carrying one electrostatic unit of quantity of electricity of either sign. Because of the close proximity of the flange assemblies to the under-vessel workers doing routine maintenance and replacement work, radiation dose rates may range from 0.3 to 0.7 rem per hour to the whole body, which is defined as the trunk, head and lens of eyes, of each worker.
The U.S. Nuclear Regulatory Commission has issued regulations governing the operation of all nuclear facilities. These regulations appear at Title 10 of the Code of Federal Regulations. In Part 20, Section 101, Paragraph (a), 10 CFR 20.101(a), the regulations currently restrict occupational doses to the whole body of a worker to a standard of 1.25 rem per calendar quarter, although the concept of `dose banking` as that term is used in 10 CFR 20.101(b), presently permits worker exposure of up to 3.00 rem per calendar quarter under very restrictive circumstances. However, the philosophy of "As Low As Reasonably Achievable", or ALARA, as it is embraced by both the Nuclear Regulatory Commission and the nuclear industry in general, motivates most reactor operators to limit occupational doses to much lower levels.
Routine under-vessel maintenance performed during boiling water reactor refueling outages can typically require from 600 to over 1000 man-hours to complete. Unfortunately, the large dose rates which can be caused by the close proximity of the flange assemblies make it possible for an under-vessel worker to receive his entire quarterly dose allowance in just a few hours. Such workers, who have used their allowable dose limit, are not permitted to work in areas where they may be exposed to any further radiation. Under-vessel maintenance work, as currently practiced, is not only a radiological safety concern, it is also a very serious manpower management problem as well.